Nuclear Instruments and Methods in Physics Research B 88 (1994) 35-43
North-Holland
RIMI B
Beam Interactions
with Materials&Atoms
Electron emission from surfaces by impact of polyatomic ions
and cosmic dust
Rafil A. Baragiola
University of V5gSa,
Engineering Physics, Charlottesuille, VA 22901, USA
This article discusses the effect of atomic aggregation (or molecular effect) in the electron emission of solids by the impact of
particles at low velocities (<Fermi velocity). For small molecules, the depression of potential electron emission results from
resonance neutralization to deep excited levels in molecules, followed by Auger de-excitation, which competes with the more
efficient channel of Auger electron capture. The near “threshold” region in kinetic emission is analyzed and it is suggested that
reported thresholds may result from insufficient experimental sensitivity. The molecular effect in kinetic emission is discussed in
terms of interference in electron scattering and of cooperative effects due to multiple atomic collisions. The latter effects are
evident when slow, large molecules are heated when rebounding from surfaces and in the hot plasma formed during the impact of
cosmic dust on solids.
1. Introduction
Studies of electron emission (EE) from energetic
particle impact on surfaces can provide insight into
physical mechanisms
operating
during impact. They
can also give information
useful in a variety of applications; for instance, EE is routinely used to detect ions
in mass spectrometry
and fast cosmic dust in space.
Furthermore,
EE is essential in the operation
of gas
discharges and affects the charging of spacecraft
and
other bodies in space.
Recent advances in EE from low velocity ion impacts on well characterized
surfaces are described in a
recent review [l]; here we will concentrate
on the case
of polyatomic projectiles on different types of surfaces.
We will describe emission mechanisms
for impacts at
velocities
< 100 km/s. These are low compared with
electron orbital velocities or Fermi velocities in solids,
but may be faster than the speed of sound in solids,
which is an important reference for the description
of
correlated
atomic motions induced by the impact of
macroscopic particles. Hence, velocities of a few km/s
are already called hypervelocities
in the field of impact
of macroscopic
(> 0.1 pm) particles.
Two main mechanisms
eject electrons in low velocity ion impacts. Potential EE (PEE) results when potential energy of the projectile
is released
through
two-electron,
Auger processes.
It does not require a
minimum velocity to occur although it may depend on
velocity due to competition
between different de-excitation channels during the finite approach time of the
ion to the surface. In kinetic EE (KEE) the
needed to emit electrons
is provided by the
energy of the projectile and there is a threshold
velocity below which ejected electrons cannot
tected, in the absence of PEE.
energy
kinetic
impact
be de-
2. Potential electron emission
There is a threshold excitation energy required for
PEE, Ei - ZlJ, where Ei is the internal energy released
in the Auger process and U is the minimum binding
energy of an electron in the solid. The Auger process
can be an Auger capture where an electron from the
solid is captured by the ion and another electron is
excited, or Auger de-excitation
where an excited projectile relaxes to a lower state and another electron is
excited. The energy U is the work function
W in
metals, or the electron affinity plus band-gap for nonmetals at T = 0 K. For non-metals
at finite temperatures, U is the energy of filled electron trap levels with
respect to the vacuum level. PEE, being an exothermic
process, has no kinetic threshold, i.e. it can occur even
if the incoming particle has zero kinetic energy. Empirically, it is found that the PEE yield (electrons/
projectile) for atomic ions in metals is proportional
to
0.7&Y, - 2W [2]. For most singly-charged
ions, PEE
does not occur at the surface of electron multipliers or
ion-electron
converters that have been exposed to the
atmosphere,
since the relation Ei > 2U is not generally
satisfied for adsorbed molecules.
0168-583X/94/$07.00
0 1994 - Elsevier Science B.V. All rights reserved
SSDZ 0168-583X(93)E0914-3
36
RA. Baragiola /Nucl.
Instr. and Meth. in Phys. Res. L388 (1994) 35-43
Molybdenum
//
atomic
ions
yields for molecular ions on clean metals (Fig. 2). First
we note that, unlike for instance noble gas atoms,
molecules have several excited states located opposite
to the filled valence band in most metals, and can thus
be populated by resonant electron capture. Auger deexcitation of this state may not be energetic enough to
eject an electron by an Auger process. The hole left in
the valence band of the metal “bubbles
up” to the
Fermi level; i.e. it is filled by interband Auger transitions where small energy transfers
are favored [S],
which do not lead to electron emission. These processes compete
with Auger capture
to the ground
state, which is the most efficient EE mechanism.
This
competition
will depend on the velocity of the approaching ion normal to the surface, and cause emission by Auger capture to increase with ion velocity.
The reason is that the faster the ion, the more likely it
is to escape resonance
neutralization
on approach. It
will then most likely arrive as an ion to a region with
sufficient overlap between the ground state orbital and
the valence band electrons from the solid, and undergo
Auger capture. This competition
has some analogy to
that described by Hagstrum for the interplay between
Auger capture
and resonant
electron
capture
plus
Auger de-excitation
[1,9].
Another
effect lowering PEE yields is capture to
repulsive states of the molecule. If sufficient time is
allowed (i.e. for very slow ions) the energy available for
Auger capture will be lowered by its conversion
into
atomic motion. The dissociation
may be frustrated
if
the molecules
relaxes to the ground state through
Auger de-excitation
which can produce electron emission. The pathway for the internal
energy of the
molecule will depend on the time it spends in the
F
-
/
/
0
7
-
P
0
L
molecular
ions
-
1
I.P. - 2W (eV;O
Fig. 1. Electron
emission yields from clean MO bombarded
with atomic and molecular ions versus the potential energy
available for emission. 0 Hagstrum [3]; A, o Vance [6]. IP is
the ionization potential of the neutralized projectile and W
the work function of the metal.
2.1. PEE
by small molecular ions
For a given Ei, PEE yields y are much smaller for
molecular than for atomic ions (Fig. 1) [3-71 and have
a strong velocity dependence
(compared
to the near
invariance
seen in the yields for monoatomic
ions).
This molecular effect in PEE has remained essentially
unexplained.
We envision two reasons for smaller PEE
E
”
%
Fig. 2. Schematic electron potential energl diagram for an incident molecular ion close to a surface. In (a) the incident ion,
approaching the surface, can be neutralized by resonant electron capture to an excited state (l), or by Auger capture (2) resulting
in electron emission. At a later time, shown in (b), the energy released in Auger de-excitation (3) may not suffice to eject an
electron. The hole produced in the band “bubbles up” to the Fermi level E, through very shallow electron excitations (4). E, is
the vacuum
level and W the work function.
R.A. Baragiola/Nucl. I&r. and Meth. in Phys.Res. B 88 (1994) 35-43
proximity of the surface. If the molecule is slow, it will
evolve along the repulsive path leaving less energy
available for Auger de-excitation, thus giving a lower
electron yield. As the velocity of the molecule is increased, it will reach the surface with higher electronic
excitation energy and the electron yields should increase, as observed. Therefore, the velocity dependence of the yields depends on the ratio between the
transition rates for Auger processes and the evolution
of the repulsive molecular state. It must be pointed out
that the literature on molecular ion impact should be
analyzed with care, since most experiments do not
control the excitation energy of the ion which affects
PEE strongly (e.g., the electron yield for N: on clean
MO increases with velocity for ground-state ions and
decreases for excited ions [6]). Other sources of experimental problems are the presence of neutrals in the
beam, which eject electrons but do not contribute to
measured beam currents, energetic ions from the fragmentation in flight of the molecule during acceleration,
and secondary emission from nearby surfaces by fast
particles ejected from the target.
2.2. PEE by large, highly charged particles
Studying light flashes produced during impact of
3-4 pm particles, Martynov [lo] found that, at velocities below 1 km/s, the light intensity is independent of
velocity and increases with the charge of the particle
(in the range 50-300 fC). The author proposed that
this can be explained assuming that light results from
an “electrical discharge” between the particle and the
surface, due to field emission. Later experiments by
Smith and Adams [ll], however, showed a complete
lack of correlation between the ion and electron emission yield and the incident charge in the range 10-1000
fC, for 0.3-6.5 pm iron particles at velocities between
0.05 and 10 km/s. These two experiments were conducted on unclean metal surfaces.
Recently, Mahoney et al. [12] explained the projectile charge dependence of EE by, assuming a stepwise
Auger capture process as is believed to occur for
incident multiply-charged atomic ions [l]. Further work
is needed to include specific molecular aspects which
should inhibit PEE, e.g., resonant electron capture
followed by relaxation of the excitation in non-electron
emitting channels, as discussed above.
on two excitation mechanisms that can occur for low
velocity projectiles:
1) Binary collisions of “free” valence electrons of
the targets with the screened Coulomb field of the
projectile, noticeable for light projectiles (H, He, Li)
on metals. The electrons can gain up to twice the
projectile velocity after a single scattering, resulting in
an energy gain T = 2mu(v + ve), where v and v, are
the velocity of the projectile and the target electron
before the collision and m the electron mass.
2) Binary atomic collisions. Electrons can absorb a
much larger energy transfer (even a substantial fraction
of the center-of-mass energy in the collision) if they are
not free, but bound to atoms. In this case, the treatment of the collision is similar to that of ionization in
gas-phase collisions, where electronic excitations occur
due to electron-electron
interactions which promote
electronic levels directly into the ionization continuum
or through autoionizing states [14]. Atom-atom collisions can excite electrons from the projectile or the
target and are thought to be the main mechanism
ejecting electrons during impacts on gas-covered surfaces [15]. The atomic collisions considered here can
occur between a projectile atom and a target atom,
between a fast target recoil and another atom, or
between atoms in the projectile in the collision spike
formed on impact.
3.1. KEE thresholds - single atoms
KEE yields decrease at low velocity and a minimum
velocity or threshold occurs below which EE cannot be
detected. This has great practical importance since it
determines the limit of sensitivity for detecting large
molecular ions in mass spectrometers operating with
constant acceleration
voltage. Different threshold
mechanisms are described below.
3.1.1. Binary ion-electron collisions
Considering the valence electron of a metal target
to act as free, one can obtain a threshold for KEE by
binary ion-electron
collisions by requiring that the
maximum energy transfer be equal to the work function W. This occurs in a head on collision with the
fastest free electrons (those with the Fermi velocity,
v,), giving the “free-electron threshold” [16]:
vth
3. Kinetic electron emission mechanisms
The description of KEE is usually done with a
three-step model, separating the steps of excitation,
electron transport (multiplication and attenuation) and
transmission through the barrier. These steps are discussed in a recent review [13]; we will concentrate here
37
=T llJF
k1+ 2W/mv$)1’2
- 11)
or about 150-300 km/s for most metals, consistent
with extrapolation of the experimental results for light
projectiles [16]. These values are, however, an order of
magnitude larger than measured threshold velocities
for heavy projectiles, suggesting either that the free
electron model is not applicable to them, or that there
is an additional mechanism.
38
RA. Baragiola/Nucl. Instr. and Meth. in Phys.Res. B 88 (1994) 35-43
A lower threshold than Eq. (1) occurs if the electron undergoes an additional scattering with the projectile, after being reflected by the target. The probability of this process is expected to increase with the
atomic number of the projectile and target atom. A
lower threshold also results if the collision occurs with
a bound electron, since then the nucleus can participate in the momentum exchange. This is the situation
that normally occurs during photoelectron emission
from solids, which otherwise is not allowed in the
scattering of a low energy photon with a free electron.
Xe
2.0
.
I
.
I
on Au
I
100:
oO_
a0
00
c
1.5 -
gE
.
gg
0
10-l:
3or
0
1.0-
0
-
.
_.
$s
.
ifj
(2)
not too close to uth which appears to agree with
expectations based on Firsov’s model [19]. The extrapolated value of uth is - 45 km/s for gas-covered, electron multiplier surfaces [20,21], roughly independent of
the type of bombarding ions. The implicit acceptance
of Eq. (2) has influenced the way data have been
plotted in the literature. One can ask whether the
extrapolated value uth is affected by a limited experimental sensitivity or the way the data are plotted. We
examined KEE induced by Xe+ ions on atomically
clean gold [22] and found that the yields depart from
the linear behavior of Eq. (2) and do not have a
definite threshold down to u = 30 km/s (Fig. 3). These
results were confirmed and extended down to u = 12
km/s by Lakits et al. [23]. KEE down to 15 km/s have
previously been observed by Waters for Cs on W [24]
who also found that gas adsorption increases the elec-
;
0
.
:
10-2:
0
.
. 0
0.5 -
.
c&f8
103;
‘.
Y = Yo( u - %I)>
:
08
.
.
3.1.2. Binary atomic collisions
A direct demonstration of the importance of excitations in atomic collisions below the free electron
threshold of Eq. (1) was made by Rabalais et al. [17]
who studied the impact parameter dependence of Ar+
induced KEE from Ni(ll0) at 4 keV and found that
emission started abruptly when the mtnimum impact
parameter in a collision was below 0.3 A.
For binary collisions between an atom in the projectile and one in the target, the minimum possible
threshold for KEE allowed by energy conservation is
when the center-of-mass energy equals U. For low
work function surfaces, like those of alkalis, one can
derive threshold velocities of around 5 km/s for oxygen projectiles, the heaviest typical component of a
large organic molecule. Even lower uth can result when
bombarding an insulator with populated electron traps
near the vacuum level. Experiments have shown smaller
+, for insulators, but no studies have been made on
the role of traps which could be characterized by
exoemission [18].
Early measurements using atomic ions have shown
that, at low velocities, the KEE yields have a dependence
i
0
0
iw
200
Velocity (km/s)
300
10-4
10
.
l
c
,
,..***’
100
’
Velocity (km/s)
Fig. 3. Electron emission yields for Xe+ on clean Au. o
Alonso et al. [22], l Lakits et al. [23].
tron yields by three orders of magnitude at these low
velocities.
The model of KEE resulting from binary low velocity atomic collisions suggests that the projectile velocity
be measured in terms of the characteristic velocity
~,=a
AE/h,
(3)
which follows from Massey’s adiabatic criterion [25],
where a is the range of the interaction causing the
transition with energy defect AE. At low velocities,
this suggest a similar dependence as have been proposed for ionization collisions in the gas phase, using a
straight-path approximation 126,271:
Y = y. exp(-u,/u),
(4)
or a sum of exponentials if several processes are present with different values of u,. Breaks in the plot of
log y vs l/u could then be used to identity different
excitation channels. Similar exp( - 0,/u) behaviors result from other low velocity processes like electron
transfer at surfaces [28]. Eq. (4) can be fitted to the
linear form (Eq. (3)) y = 0.541yo u;’ (U - iuc), over a
wide range around the inflection point at uJ2.
In the limit of very low velocities Eq. (4) should
break down for at least two reasons: 1) the inability to
reach interatomic distances where the coupling to the
continuum is sufficiently strong, and 2) the existence of
an absolute binary threshold when the center of mass
energy equals U. Situations close to this absolute binary threshold have been identified in the impact of
relatively light ions on gas covered surfaces [15], from
comparisons with gas-phase ionization cross sections.
Beyond the binary collision approximation, the minimum possible threshold results from energy conservation, when the projectile energy equals U. For this to
occur, the lattice has to absorb all the momentum of
the projectile and all the available energy has to go
into the excitation of a single electron. This is very
unlikely for heavy particle impact, especially for polyatomic projectiles, since there is a large number of
alternative pathways for the dissipation of the incident
energy.
3.2. Molecular effects in kinetic electron enaission
3.2.1. Illzresholds
A first approximation to electron emission by polyatomic projectiles is that each incident ~nstituent atom
behaves independently
and so it contributes with its
individual threshold. Thus, tith for polyatomic impact
will be the smallest uth of its atomic components. Since
the electron yields for polyatomic impact are larger
than for atomic ions, there is more relative sensitivity
in the measurements which may produce an apparent
lower +, than those measured for single ion impact. In
recent years, several groups have made brief explorations of the KEE threshold for gas-covered surfaces,
mainly aimed at characterizing the limit of detection of
large masses by mass spectrometers. Using large H,O
clusters, Beuhler and Friedman could measure eleetron yields down to v = 18 km/s from a gas-covered
Cu surface 1291and 9.5 km/s from an ~uminum oxide
target [30]. Extrapolated
thresholds below 20 km/s
were also obtained for organic molecules by several
groups [31-341. To the knowledge of the author, the
lowest velocity for which electron yields have been
measured, is 7.5 km/s for the organic molecule bovine
trypsin (MW = 23295) on CsI [35] while electron signals have been seen down to - 7 km/s for albumin
molecules (MW - 66000) on a stainless steel target
[36] and from heated, C,, molecules rebounding from
surfaces (see below). Work by Even et al. [37] on
impact of seeded, supersonic CQ, molecular beams on
surfaces at 1.6 km/s suggests even lower threshold
velocities. However, the study did not distinguish between electrons and negative ions which have been
reported in similar experiments by Christen et al. [38]
using 1.2 km/s SOZ clusters.
3.2.2. Yields
We now discuss how the electron yields produced
by molecular ions compare to those produced by their
constituent atoms. It is found that for hydrogen projectiles of equal velocities, the yield ratio R, =
r(Hz, u)/ny(H+, v) < 1 at low u while, for H,, R, > 1
at u higher than about 5000 km/s [39-421. The reduction at low velocities occurs even after taking into
account the smaller PEE yield for molecular ions discussed above. The depression of KEE yields is very
strong for H clusters and R, is observed to reach a
plateau of N 0.35 for IE> 9 at 30 keV/amu [43,44].
The origin of this molecular effect has been a
matter of controversy. We have argued for an interfer-
enee effect, [39] which has been demonstrated for
electronic stopping powers: molecular hydrogen ions
lose less energy than protons at low v and more at high
v, due to an interference in the scattering of the target
electrons in the molecular centers [45]. At velocities
close to the threshold, only the largest momentum
transfers (- 2hkik,) are important, where hk, is the
electron momentum at the Fermi surface. The molecular effect for H, will then be
R, = I+ sin(2+)/2k,r
(5)
near threshold, where I is the inter-proton distance.
Thus the yields are depressed (R, < 1) for most metals,
as observed (2kfr between 3.2 and 5) and should be
enhanced for alkali targets. This description can be
expanded to large clusters following recent theoretical
work on electronic stopping of clusters [46].
An alternative proposal [471 is that the molecular
fragments act totally independent of each other: r(Hi)
= y(H+) + y(H”) (i.e., interference
effects are ignored) and explains R, < 1 at low velocities making
two assumptions: (1) that each constituent atom of the
molecule acts independency of the others, and (2) that
y(H) < r(H’).
The second assumption is based, in
turn, on two additional assumptions: (3) that y is
propo~ion~ to the electronic stopping power S,, even
at low velocities, and (4) that S, is lower for Ho than
for H+, due to the screening of the proton charge in
Ho. However, ionization cross sections at keV energies
may be larger for H than for Ht impact [4X].
The molecular effect with slow, heavy atom impact
is not clear. Thum and Hofer [49] found that the yields
are additive (R, = 1) when using Vc and Nbc clusters
impacting stainless steel. In conditions of higher energy
deposition, Veje [SO] found R, <R, < 1, using 30
keV/atom
Sbz on Au, Oliva et al. [Sl] observed
R, > 1 for keV Xet and Xeg on Au and Svensson et
al. found R, = 1 for low velocity Se+ and Tez impact
on Ag and a decrease in R, above 25 keV/atom.
Using large clusters of water molecules, Beuhler
and Friedman [52] found that the additivity of the
electron yields holds for Cu [53], but not for aluminum
oxide targets [54], where the yield for the cluster was
lower than the sum of the yields for the molecular
constituents. This non-additivity may be due to localized charging of the insulating oxide surface which
inhibits large electron yields. Such inhibition should
not be important near threshold, where yields are very
low. These results also hold when comparing different
molecules, rather than clusters of different sizes composed of the same molecules. The yields are found to
grow linearly with projectile mass for a graphite target
but sublinearly for Al,O, 1551.
A depression of the KEE yields is expected for
incident molecules with size comparable to electron
mean free paths since in this case electrons excited at
40
R.A. Baragiola /Nucl.
Instr. and Meth. in Phys. Res. B 88 (1994) 35-43
the leading edge of the impact can be attenuated if
their motion is through the molecule [56]. This should
also depend on the structure of the molecule (e.g.,
globular vs linear) and the orientation during impact.
The orientation of the molecule could also have an
effect in the statistics of electron emission.
4. Enhanced electron emission by clusters due to cooperative effects
We discuss here deviations from additivity in the
yields for polyatomic impact. Mechanisms include acceleration of atoms due to collisions between atoms in
the early times of the collision cascade (Fermi shuttle
mechanism), and thermoionic emission from a “thermal spike” created by the impact. Both electron and
ion emission can be enhanced by nonlinear or “hot
spike” effects, produced when a large and energetic
molecule is stopped, depositing most of its energy in a
very small region of a solid. Due to multiple collisions
between moving atoms during impact, some of these
atoms will acquire velocities larger than the beam
velocity, making electron emission more likely. High
energy tails in the energy distribution of atoms in large
clusters bombarding solids have been seen in molecular dynamic simulations [57] but not yet measured.
These high energy tails can cause Auger emission from
inner-shell hole decay, for incident cluster ions at subthreshold velocities (i.e., below that needed in single
atom impact) [58].
An unambiguous identification of thermal spikes
may require velocities low enough to be below the
absolute threshold for KEE by an individual atom of a
molecule, or below N 5 km/s for biomolecules containing atoms not heavier than oxygen. A particular
case is that of molecules bouncing from surfaces at low
velocities. Once in vacuum, if the internal energy gained
during impact deformation cannot be lost efficiently
through fragmentation, it will be distributed in lattice
vibrations. The rebounding molecule will evolve towards thermal equilibrium and may cool down by ejecting an electron through the thermoionic effect. It has
been proposed that this is the mechanism responsible
for delayed (us range) ionization after C, clusters
have been heated by lasers [59], by gas phase collisions
[60], or by surface collisions. Yeretzian and Whetten
[61] found that reflection of C& ions from graphite
above 170 eV (6.7 km/s) is accompanied by delayed
electron emission. They estimate that 25-30% of the
impact energy is transferred to internal energy of the
cluster. No delayed emission was observed on scattering off Si surfaces. St. John et al. [62] studied the
impact of CL and Si, clusters on Si(ll1) and graphite
surfaces; below 10 km/s they observed the intact reflected ion and its charged fragments, along with de-
layed electron emission from the intact reflected negative ion. This emission competes with fragmentation as
a way of relaxing the internal energy of the reflected
ion.
Thermoionic processes in rebounding molecules may
explain electron or negative ion emission seen using
seeded supersonic beams near 1 km/s, mentioned
above [37,38]. Electronic excitations in thermal spikes,
causing carrier excitation in semiconductors, have been
demonstrated by Cardillo et al. [63] for Xe projectiles
at velocities as low as 0.4 km/s. The only unambiguous
case of electron emission from a heated impact region
is that of surface collisions of cosmic or laboratory
accelerated dust particles, where they are seen to produce micro plasmas at velocities as low as 0.08 km/s, a
subject that is discussed below.
5. Electron emission from cosmic dust impact
Fast micrometeoroids or energetic dust particles
can be detected by stopping them on a solid target,
where they produce the emission of positive and negative charge, acoustic waves and craters [64]. Dust detectors exploiting these effects have been flown in
several spacecraft [65,66] and used to detect particles
of sizes down to a few thousand A. The effect of the
smallest particles is of special interest here since they
approach
the size of the largest clusters and
biomolecules used in the field of polyatomic impact
(they are still an order of magnitude larger). The size
of the particle producing the crater can be obtained
through laboratory simulations [67] using, e.g., electrostatic accelerators. There are many empirical formulas
relating the penetration depth 3 to impact conditions
[68]. One that is frequently used can be approximated
by:
a= CE; ( P~/P,)““/~~~‘“,
(6)
where C is a constant, E i the component of the
energy of the projectile along the surface normal, pt
(p,) is the density of the target (projectile) and gt the
tensile strength of the target, with n = 0.33-0.44. The
penetration of particles in the mass range 10-14-1 g,
described by this formula, is quite different from that
of slow atomic particles, which is roughly independent
of mass, for a constant velocity.
During the impact, a fraction of the projectile and
the target is transiently heated to high temperatures.
As a result, the impact region emits light and partially
evaporates. The temperature of the hot region has
been estimated to increase from 2500 K at 4 km/s to
5000 K at 20 km/s from measurements of the spectrum of the emitted light [69]. A part of the evaporated
flux is ionized; thermoionic electron emission occurs
R.A. Baragida /Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 35-43
according to Richardson’s equation, while ion emission
follows the Saha-Langmuir
equation. Mass analysis of
these ions can be used to identity the components of
the impacting particle [70]. The collection of the charge
liberated on impact is used to detect and identity dust
particles by space probes [6.5,66], with a sensitivity of
about lo-l6 g. Both the mass and the velocity of the
particle can be obtained by measuring the magnitude
and the rise time of the detected current pulse.
A threshold velocity of - 0.8 km/s for EE was
proposed by Dietzel et al. [71] for Fe projectiles on
different metal targets. They state that, below this
value, the particle bounces semi elastically and neither
cratering nor plasma emission is observed. However,
using a more sensitive electron multiplier detector,
Smith and Adams [ll] could detect ejected ions with
velocities down to 0.05 km/s using Fe particles of mass
between lo-l3 and lo-’ g impacting MO. The experimental arrangement did not allow electron detection
below - 0.7 km/s but it was determined that the
10
100
Velocity (km/s)
Fig. 5. Charge emitted per unit mass for accelerated dust
particles of different materials impacting a gas covered Au
target. The inflection of the curves suggests the existence of
different mechanisms adapted from ref. [77].
electron yield was roughly equal to the ion yield for
velocities between this value and - 5 km/s. The total
105
amount of charge ejected from the impact plasma has
been fitted, over limited velocity ranges, to a form
Q*/m
103
s
102
3
IO'
22
100
P
a
10-I
f
1
103
104
e
16'
41
10-2
10-3
10-4
10-5
.- 0.01
0.1
1
10
100
Velocity (km/s)
Fig. 4. Charge emitted per unit mass for accelerated Fe dust
particles impacting gas covered W (Dietzel et al. [711),MO
C-OSmith and Adams [ll]); Au (. . . . . . GGller and Griin
[77]). Also shown are electron yield data for impact of large
molecules on gas covered surfaces: C&a (0) and trypsin: (A)
on CsI by Brunelle et al. [33] and different biomolecules on
Also, (+ ) by Reimann et al. [781. The velocity scale can be
converted to energy per unit mass in eV/amu as E/m =
0.00518u2, where u is in km/s. 1 C/kg = 1.04~ lo-’ electrons/amu.
= kuP,
(7)
where m is the mass of the projectile and u its velocity.
Most experiments find p = 2.6-3.5 and some find a
weak additional mass dependence of Q/m.
There have been several attempts at modeling this
behavior using shock waves and thermoionic emission
from transiently heated regions in the projectile or
target [11,72,73,74]. Fig. 4 is a compilation of the
charge of the emitted plasma particles, taken by different groups using electrostatic accelerators. The behavior of Eq. (7) is obeyed roughly, with deviations due to
differences in experimental conditions. For instance,
Friichtenicht [75] found that Q depends on the type of
target, increasing along the sequence: In, Cu, Be-Cu,
Pb, W, Pt and Ta with the last two showing a larger
exponent p. The first targets showed also much larger
craters than the more refractory W, Pt and Ta. Timmermann and Griin [76] found that lo-”
to 10-i’ g
projectiles produce plasma yields hundred times smaller
in ice than in gold. All the dust impact experiments
have been done with “technical” surfaces with unspecified contaminant layers.
A closer look at Q/m over a limited velocity range
shows a more complicated dependence than that given
by Eq. (7) (Fig. 5) [65,77]. The inflection in the curves
suggests the existence of different mechanisms, each
one dominating in a particular velocity range. Similar
behavior is also seen in photon emission during impact
[69]. A possibility is that the low velocity behavior is
governed by thermoionic emission from rebounding
42
R.A. Baragiola /Nucl. Istr.
and Meth. in Phys. Res. B 88 (1994) 35-Q
projectiles, followed at higher velocities by thermoionic
emission from the impact region. At high velocities,
electron emission may occur by independent atomic
impacts, since it is found that for u > 10 km/s, Q/m is
comparable to that produced by polyatomic ions of
much smaller mass (Fig. 4).
6. conclusions
In many cases, electron emission from solids by
large molecules can be explained as the superposition
of emission by each of their constituent atoms. Lower
potential emission yields than expected occur for impact of small molecules, due to the dissipation of
potential energy in low lying excitations in metals or to
its conversion to internal atomic motion. A large depression of the yields has been seen in the impact of
clusters of hydrogen molecules and in the impact of
biomolecules on non-conducting
targets. When slow
molecules bounce from surfaces, the energy of deformation may be stored in vibrational motion, instead of
being relaxed in molecular break-up. The hot rebounding molecule may then relax through thermoionic emission. Electron emission from hot spikes in the solid,
often sought for in studies of molecular impact has not
yet been identified, except for macroscopic (2 1 pm)
dust particles in space and laborato~ experiments.
One of the central questions in the field of polyatomic
impact is to delimit the transition region from conditions where superposition prevails to those where hot
spikes effects dominate.
The author acknowledges stimulating discussions
with N.R. Arista, W. Ens, R.E. Johnson, C.T. Reiman,
and B.U.R. Sundqvist. This work was supported by
NSF through grant DMR-9121272 and by NATO
through grant CR~90~013.
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